Fractional rumen passage rates (K1) are fundamental in feed evaluation systems for ruminants to predict the extent of nutrient degradation. Data on passage kinetics of growing beef cattle fed high-concentrate diets are scarce and mainly rely on external passage markers which do not provide nutrient-specific K1 estimates. The present study describes the use of carbon stable isotopes (d13C) as an internal marker to estimate K1 of dry matter (DM) and neutral detergent fibre (NDF) fractions of a compound feed in a high-concentrate diet, and compares them to the external markers ytterbium (Yb)-actetate and chromium mordanted fibre (Cr-NDF). Four rumen-fistulated Holstein heifers received four times per day a basal diet consisting of barley straw and pelleted compound feed offered separately (ratio 10 : 90, DM basis). Compound feed in the basal diet was mainly based on wheat of low natural 13C enrichment (-28.4 d13C), which was exchanged with a single dose of a maize-based compound feed of higher natural 13C enrichment (-18.9 d13C). This difference in natural 13C abundance was used to determine K1 values from faecal 13C excretion patterns. At the same time Yb-Acetate and Cr-NDF were introduced into the rumen to determine K1 values from faecal excretions. Faeces were collected over 90 h after pulse dosing. The K1 of d13C-marked DM (0.062/h) did not differ (P = 0.745) from d13C-marked NDF (0.060/h). The d13C-based K1 values also did not differ from Cr-NDF (0.056/h; P = 0.315). These results indicate similar passage behaviour of these fractions in the rumen of beef heifers fed a high-concentrate diet.
- different forage